1. Field of the Technology
The present disclosure is directed to forging methods for titanium and titanium alloys and to apparatus for conducting such methods.
2. Description of the Background of the Technology
Methods for producing titanium and titanium alloys having coarse grain (CG), fine grain (FG), very fine grain (VFG), or ultrafine grain (UFG) microstructure involve the use of multiple reheats and forging steps. Forging steps may include one or more upset forging steps in addition to draw forging on an open die press.
As used herein, when referring to titanium and titanium alloy microstructure: the term “coarse grain” refers to alpha grain sizes of 400 μm to greater than about 14 μm; the term “fine grain” refers to alpha grain sizes in the range of 14 μm to greater than 10 μm; the term “very fine grain” refers to alpha grain sizes of 10 μm to greater than 4.0 μm; and the term “ultra fine grain” refers to alpha grain sizes of 4.0 μm or less.
Known commercial methods of forging titanium and titanium alloys to produce coarse (CG) or fine grain (FG) microstructures employ strain rates of 0.03 s−1 to 0.10 s−1 using multiple reheats and forging steps.
Known methods intended for the manufacture of fine (FG), very fine (VFG) or ultra fine grain (UFG) microstructures apply a multi-axis forging (MAF) process at an ultra-slow strain rate of 0.001 s−1 or slower (see G. Salishchev, et. al., Materials Science Forum, Vol. 584-586, pp. 783-788 (2008)). The generic MAF process is described in C. Desrayaud, et. al, Journal of Materials Processing Technology, 172, pp. 152-156 (2006).
The key to grain refinement in the ultra-slow strain rate MAF process is the ability to continually operate in a regime of dynamic recrystallization that is a result of the ultra-slow strain rates used, i.e., 0.001 s−1 or slower. During dynamic recrystallization, grains simultaneously nucleate, grow, and accumulate dislocations. The generation of dislocations within the newly nucleated grains continually reduces the driving force for grain growth, and grain nucleation is energetically favorable. The ultra-slow strain rate MAF process uses dynamic recrystallization to continually recrystallize grains during the forging process.
Relatively uniform, cubes of UFG Ti-6-4 alloy can be produced using the ultra-slow strain rate MAF process, but the cumulative time taken to perform the MAF can be excessive in a commercial setting. In addition, conventional large scale, commercially available open die press forging equipment may not have the capability to achieve the ultra-slow strain rates required in such embodiments and, therefore, custom forging equipment may be required for production-scale ultra-slow strain rate MAF.
Accordingly, it would be advantageous to develop a process for producing titanium and titanium alloys having coarse, fine, very fine or ultrafine grain microstructure that does not require multiple reheats and/or accommodates higher strain rates, reduces the time necessary for processing, and eliminates the need for custom forging equipment.